Keying system



Oct. 16, 1945. R, DAVIS 2,386,844

KEYING SYSTEM Filed Aug. 10, 1942 T0 RECEIVER Fig.1

. INVENTQR ROBERT J. DAV/5 ATTORNEY Patented Oct. 16, 1945 KEYING SYSTEMRobert J. Davis, Lutherville, Md., assignor to Bendix AviationCorporation, South Bend, 11111., a corporation of Delaware ApplicationAugust 10, 1942, Serial No. 454,313

6 Claims.

This invention relates to radio transmitters emitting keyed signals andmore particularly to an improved keying system for use in suchtransmitters.

Present day radio transmitters employ radio frequency amplifiersoperating with very strongly negative control grid bias to secure goodefiiciency. A variety of methods are utilized to secure the necessaryoperating bias, such as the use of a separate source for the biaspotential, the.

use of a portion of the anode supply voltage and the use of the gridcurrent resulting from the application of excitation to the control gridof the power amplifier stage passing through a grid resistor to developthe required bias. In transmitters designed for use in aircraft andmobile vehicles the latter method is most frequently employed as itaffords the maximum output energy from the amplifier stage for a givenanode supply voltage, thereby diminishing the weight of the anode powersupply necessary to provide a given radiated energy. The use of thissystem is characterised by the complete-loss of power amplifier controlgrid bias during those portions of the excitation keying cycle duringwhich no signal is present. This reduction in bias shifts the tubeoperating point to a region of the characteristic in which the mutualconductance is relatively high, and as a result, a stage which is stableunder all other normal operating conditions may oscillate parasiticallyand emit radiation during the 011' period of the keying cycle. Becauseof the weight and space limitations in aircraft transmitters, it is notpossible to make use of a separate source of negative grid bias to avoidoperation in this portion of the characteristic and it has thereforebeen the practice to key the high voltage supply to the anode of thepower amplifier or to key the connection between the cathode of thepower amplifier and the negative terminal of the anode highvoltage'source. This remedy was-reasonably satisfactory except inaircraft operating at altitudes of 30,000 to 40,000 feet where excessivearcing and sparking of the keying contacts was observed leading to theirearly destruction. At these altitudes, the insulating property of theair is markedly impaired and the anode circuit carrying potentials of1000 volts or more cannot be readily interrupted.

A principal object of this invention is to provide new and novel meansfor the suppression of undesired radiation during the no signalintervals of a keyed radio transmitter.

Another object of the invention is to provide a system for thesuppression of spurious radiation from a keyed radio transmitter whichoperates satisfactorily and without damage to the contact points underthe low atmospheric pressures found at high altitudes.

The above objects and advantages are accomplished by tuning the inputand output circuits connected to a power amplifier to different resonantfrequencies during the off period of power amplifier excitation, andtuning said circuits to the same resonant frequency during the on periodof power amplifier excitation.

Other objects and advantages of the invention will in part be describedand in part be obvious when the following specification is read inconjunction with the drawing in which the single figure is a schematicdiagram of a radio transmitter embodying the invention.

Referring further to the figure, the cathode l of the oscillator tube 2,which may be of the type commercially designated as 807, is connected tothe tap on the fixed winding 3 of the variometer I 6 and one end of thewinding 3 is connected to ground. The other end of winding 3 isconnected to one end of the movable winding 5 of the Variometer 4 andthe other end of the winding 5 is in turn connected to the tuningcapacitor '6 in series with the loading inductance 1. This terminal ofcondenser 6 is also connected to the control grid 8 of vacuumtube 2 inseries with the parallel combination'of grid leak 9 and capacitor l0 andthe other terminal of capacitor 6 is connected to ground. The spacecharge grid l l is supplied with a positive potential from the 380-volttap on the 1000-volt direct current source l2 through the droppingresistor l3 and the contact 2| on the keying relay 22, and the grid H ismaintained at ground potential for radio frequency currents by thecapacitor l4 connected between grid II and ground. The voltage regulatortube l5 which may be of the type designated as VR- is also connectedbetween space charge grid l l and ground to stabilize the potential ofgrid l l and prevent frequency variations with changes in the outputvoltage 'of source l2. The suppressor grid it of tube 2 is connected tothe cathode I and the anode ll of tube 2 is connected to the 380-volttap of source l2 in series with the parallel resonant circuit includingcapacitor I8 and inductance I9, the isolating resistor 20 and thecontact 2| on the keying relay 22. The decoupling action of resistor 20is enhanced by the capacitor 23 connected between the anode side ofresistor 2i! and ground. The cathode l is maintained at operatingtemperature by the heater 24 connected to the filament voltage source 25through the isolating chokes 26, 21. The capacitors 28 are connected inseries across the terminals of heater 24 and their junction point is inturn connected to the cathode I to minimize the radio frequencypotential difierence between the cathode I and associated heater 24. Thenegative terminal of source 2 5.is groundedfor direct current andrthepositive terminal of this source is grounded for radio frequencypotentials by the capacitor 29 connected between this point and ground.The circuit just described in connection with oscillator tube 2 is aHartley oscillator circuit and its operation will not be dwelt upon indetail because of its familiarity to those skilled in the art. Sufiiceit to say that the oscillation involves only the cathode I, control grid8 and the space charge grid I I and that its frequency is controlled bythe resonance frequency of the circuit including the variometer 4 andthe capacitor '6. This frequency is selected by variation of therelative positions of the coils 3 and'5'in the variometer4.

The current flowing to the anode flows in pulses of the same frequencyas the oscillatingpotentials appearing on the control grid 8 anddevelops a voltage across the resonant circuit including capacitor I8and inductor I9. As the anode load impedance does not control thefrequency of oscillation, this circuit maybe 'tuned to the fundamentalfrequency of oscillation or to any desired harmonic thereofaccordingtothe desired output frequency. Voltages appearing across thisload circuit are'impresse'd on the control grid 30 of the poweramplifier3| which may .be of the commercial type 814, through the couplingcapacitor 32. A 'direct current path from the control grid .30 to ground'isestablished through the choke 33 and resistor 34 connected betweenthis point and ground. The rate .of change of the operating bias ofamplifier 3| is controlled by the capacitor 35 connectedin'parallel'with the resistor 34 and the .choke 33 prevents excessiveloss of driving power through this portion of the circuit. Operatingemission for the amplifier 3| is provided by the filament 36 having oneterminal grounded and the other connected to the positive term'inal ofsource'25. "Situated on the other side of the control grid'30from thefilament '36 is'the space charge grid 31 maintained at a positive directcurrent potential by connection to the fixed contact 2| of relay 22through the dropping resistor 38 and maintained at ground potentialfor-radio frequency currents by the capacitor 39 connected between grid31 and ground. The suppressor 40 is connected to ground and anode 4| isconnected to the 1000- volt terminal of source I2 through the choke 42.Potentials appearing across the choke 42 are impressed on thetank'circuit .including the variable inductor 43 and variometer winding'44 connected in series across the capacitor 45 through the blockingcapacitor 46 connected between the anthe variometer secondary 41 wherethe coupling 49b and the rear contact 490.

between the tank circuit and the antenna circuit is effected. Inoperating the equipment, the loading inductor 48 is adjusted to bringthe antenna circuit into resonance with the output frequency and therelative position of the coils 44 and 41 in the variometer is adjustedto provide the most satisfactory reflected load impedance for the poweramplifier 3|. When keying relay 22 is deenergized,-the antenna'50 isconnected to any desired receiving equipment via the movable contact Thecontacts 5|a and 5|b are also controlled by the keying relay 22 .forapurpose to be more fully described. Movable contact 5|a is connected toground while fixed contact'5lb'is connected to the anode I! throughthe-capacitor 52 which is thus effectively in parallel with capacitor I8when contacts 5| are closed, due'to the negligible impedance ofcapacitor 23 at the operating frequency. In a particular transmitterwhere capacitor I8 had a value of approximately mmfd., the capacitor 52was made .003 mfd. withvery satisfactory results. One end of theoperating winding of relay 22 is connected to the positive terminal ofthe source 25 and the other terminal of this winding is connected toground in series with the key53. Closureof key 53 excites the operatingwinding of relay 22 closing contacts 2| ,opening-conta'cts 5Ia and -5Ibandmoving the movable contact 4912 into-engagement with thecorresponding front contact 48a. With the key 53-open, contacts 2| areopened, contacts 5|a and -5|b are closed, and movable contact 49bengages the corresponding rear contact 49c.

Thus, with the key 53 down, anode and screen voltages are applied to theamplifiers 2 and BI and the antenna 50 is coupled to the transmitteroutput. The circuit now operatesas a conventional oscillator and poweramplifier with the grid current flowing duringthe, positivesignal, peakson the control grid 30 supplying operating bias for the power amplifierby virtue of the :voltage drop across the resistor 34.

'With key 53 up, however, the screen andanode of oscillator 2, and thescreen of the .power amplifier 3| are disconnectedfrom the highvoltagesource, and the antenna50 is connected to whatever receiving equipmentis used in the installation. When only these modifications are performedwhen the key is up, it has been found that, despite the absence ofpotential on screen grid 31, both parasitic and tuned-grid tunedplatemodes of oscillation occur in the power amplifier stage as the grid biason grid 30 vanishes due to the removal of excitation. This is because ofthe very high anode potential which causes the continuance of anodecurrent flow in theab- ,sence of screen potential. In the arrangementof'the'invention, however, contacts 5| ground one terminal of thecapacitor 52, thereby detunin the resonant circuit 18, IS in the gridcircuit of the power amplifier 3| from the frequency of theresonant'circuit 43, 4'4, 45 in the anode circuit of the power amplifier3|, and no oscillations appear due to'the removal of the conditionsnecessary for their existence. The contacts 5| which perform thisfunction are not subjected to the high potentials existing acrosssimilar elements in previously known systems for the suppression of suchoscillations and their life expectancy under high altitude operatingconditions has been found considerably greater than that of contactsrequired to break circuitsin which operating potentials are 1000 voltsand more.

It will be obvious that'many changes and modifications may be made inthe invention without departing from the spirit thereof as expressed inthe foregoing description and in the appended claims.

I claim:

1. In a radio transmitter, an electric discharge device having acathode, a control grid and anode, a resonant circuit tuned to apredetermined frequency connected between said cathode and said controlgrid, means for intermittently impressing alternating current potentialson said control grid, means actuated by the current flowing to saidcontrol grid for developing control grid bias, a second resonant circuittuned to said predetermined frequency connected to said anode, and meansconnecting a capacitor across said input resonant circuit during theabsence of alternating current potentials from said control grid.

2. In a radio transmitter, means for converting direct current energy toalternating current energy having a predetermined frequency, a resonantcircuit tuned to said predetermined fre-' quency connected in the outputcircuit of said conversion means, an electric discharge device having acathode, a control grid, a space charge grid, and an anode, meansconnecting said resonant circuit to said control grid, a source ofdirect current energy, a second resonant circuit tuned to saidpredetermined frequency connected between said anode and said directcurrent source,

a detuning capacitor, and switch controlled electromagnetic relay meansintermittently connecting said conversion means and said space chargegrid to said direct current source and connecting said detuningcapacitor across said first mentioned resonant circuit during the timethat said conversion means and said space charge grid are disconnectedfrom said direct current source.

3. In a radio transmitter, an electric discharge device having input andoutput circuits, a resonant circuit tuned to a predetermined frequencyconnected in said input circuit, a second resonant circuit tuned to saidpredetermined frequency connected in said output circuit, means forsupplying keyed excitation to said input circuit, and means detuningsaid input resonant circuit from said predetermined frequency during theabsence of said excitation.

4. In a radio transmitter, an electric discharge device having input andoutput circuits, a resonant circuit tuned to a predetermined frequencyconnected in said input circuit, a second resonant circuit tuned to saidpredetermined frequency connected in said output circuit, means forintermittently impressing alternating current potentials on said inputcircuit, and means changing the resonant frequency of one of saidresonant circuits during the absence of said alternating currentpotentials from said input circuit.

5. In a radio transmitter, an electric discharge device having input andoutput circuits, a resonant circuit tuned to a predetermined frequencyconnected in said input circuit, a second resonant circuit tuned to saidpredetermined frequency connected in said output circuit, means forintermittently impressing alternating current potentials on said inputcircuit, and means connecting a capacitor across said input resonantcircuit during the absence of said alternating current potentials fromsaid input circuit.

6. In a radio transmitter, an electric discharge device having acathode, a control'grid and an anode, a resonant circuit tuned to apredetermined frequency connected between said cathode and said controlgrid, means for intermittently supplying alternating current potentialsto said control grid, means actuated by the current flowing to saidcontrol grid for developing control grid bias, a second resonant circuittuned to said predetermined frequency connected to said anode, and meanschanging the resonant frequency of one of said resonant circuits duringthe absence of said alternating current potentials from said controlgrid.

ROBERT J. DAVIS

